Not applicable.
Not applicable.
(1) Field of the Invention
The present invention generally relates to welding methods for superalloys. More particularly, this invention is directed to a method for electron beam welding single-crystal nickel-base superalloys containing significant amounts of refractory elements, by which the incidence of cracking is reduced in the resulting welded assembly.
(2) Description of the Related Art
Nickel-base superalloys are widely used to form components of gas turbine engines, including combustors and turbine vanes and blades. Superalloy components are often formed by casting, and for some applications are preferably or necessarily fabricated by welding as a result of their complexity. Welding is also widely used as a method for restoring blade tips, and for repairing cracks and other surface discontinuities in superalloy components caused by thermal cycling or foreign object impact.
Structural welds of nickel-base superalloy castings containing gamma-prime (Yxe2x80x2) precipitates, and particularly those containing significant amounts of refractory elements such as tantalum, aluminum, molybdenum, tungsten, rhenium and niobium (columbium), are known to form strain age cracks upon cooling from welding or upon subsequent reheating, such as during aging when the gamma prime phase is reprecipitated following solution heat treatment. As an example, the single-crystal nickel-base superalloy known as Renxc3xa9 N5, containing greater than 10 weight percent refractory elements, has been generally viewed as unweldable. The cause of cracks in superalloys such as Renxc3xa9 N5 is due at least in part to the residual stress produced during the welding and aging cycles.
Low heat input welding processes, such as laser or electron beam (EB) welding (collectively referred to herein as high energy beam welding) have been used to produce crack-free weld joints in single-crystal superalloys over a narrow range of welding conditions. An advantage of EB welding processes is that the high energy density of the focused electron beam is able to produce deep, narrow welds at high speed, making possible the formation of structural butt welds that add minimal additional weight. However, a drawback observed with laser beam and EB welding processes is directional grain growth in the fusion zone, which forms a distinct dendritic boundary in the center of the weld zone. This type of grain structure makes the joint vulnerable to centerline cracking, which reduces the fatigue strength of the welded component. Another problem encountered when high energy beam welding single-crystal superalloys is associated with the use of single-crystal backing strips beneath the abutting ends of a butt weld. Both of these defects are represented in FIG. 1, which shows a welded assembly 10 comprising a pair of superalloy components 12 and 14 and a backing strip 16, in which the components 12 and 14 are joined by a butt weld joint 18. A centerline crack 20 is represented as being present in the joint 18, while a root crack 22 is shown in the backing strip 14.
High temperature TIG (tungsten inert gas) welding processes have been developed to overcome the centerline cracking problem associated with single-crystal superalloy joints formed by high energy beam welding. However, joint thickness and dimensions are limited by TIG processes, and their use is limited by the requirement for restrictive control of temperature, atmosphere and process parameters in order to produce a uniform grain structure for acceptable fatigue properties. Even then, there is the risk of excessive distortion and heat-affected zone (HAZ) cracking.
In U.S. Pat. No. 6,489,583 to Feng et al., EB welding is used in combination with a superalloy shim to form weld joints in single-crystal superalloys. Using a narrow range of welding conditions, the welding process disclosed by Feng et al. is able to avoid the formation of centerline cracks encountered by previous high energy beam welding processes. However, the high energy beam welding process disclosed by Feng et al. has not eliminated the development of root cracks where both components being welded are single-crystal superalloys, and particularly where the superalloys contain relatively high levels of refractory elements, as in the case of Renxc3xa9 N5.
In view of the difficulties discussed above, single-crystal superalloy assemblies have most often been formed from superalloys that are less prone to cracking, or assembled with fasteners or by brazing. However, the use of fasteners requires flanges and brazing typically requires a large interface (faying surface), both of which result in increased weight. Therefore, it would be desirable if a welding process existed for joining single-crystal nickel-base superalloys, particularly those containing 10 weight percent or more of refractory metals, and which was capable of producing a crack-free joint that exhibits improved fatigue life at high temperatures and strains.
The present invention generally provides a method for welding components formed of nickel-base superalloys, particularly single-crystal superalloys containing 10 weight percent or more of refractory metals, and to the resulting welded assemblies. The method of this invention reduces the incidence of cracking through the use of a high energy beam welding process that makes use of a shim and particular welding parameters to develop a defect-free joint that exhibits improved fatigue life at high temperatures and high strain ranges.
The method of this invention involves high energy beam welding single-crystal nickel-base superalloy articles, with the above-noted shim placed in a gap between the articles so that the shim contacts the faying surfaces of the articles. A backing strip may be present that contacts both articles and bridges the gap between the articles. According to one aspect of the invention, the shim may be formed of a nickel-base superalloy that is more ductile than the nickel-base superalloys of the articles. The articles are welded together using a high energy beam with a current pulse frequency of about 10 to 50 Hz and a travel speed of about 0.85 to about 1.5 cm/s. The high energy beam causes the shim and portions of the articles contacting the shim to melt, and the superalloys of the shim and articles to mix. According to the invention, the frequency and speed parameters of the welding process cause the resulting weld joint to form multiple roots that extend into the backing strip (if present).
A preferred aspect of the invention is the elimination of both centerline and root cracks in the weld joint and, in the case of a buft weld, full penetration of the weld joint. Other advantages of the invention include reducing part distortion and a simplified process in the production of superalloy joints for complex structures, including airfoils for gas turbine engine applications.
Other objects and advantages of this invention will be better appreciated from the following detailed description.